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Old 26-08-2010, 03:00 PM
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mental4astro (Alexander)
kids+wife+scopes=happyman

mental4astro is offline
 
Join Date: Jun 2008
Location: sydney, australia
Posts: 5,005
Cheap eyepieces, such as cheap plossls and wide field EPs, perform best in slow scopes, f/6 or slower. In a fast scope, they will show astigmatism, especially in an f/4.4 scope. No way around that one other than purchasing expensive EPs, from $300 +.

The shorter the focal length of these cheap eyepieces, however, the astigmatism is greatly reduced, but it becomes really hard to use these cheap eyepieces.

Tightening the mirror in the cell is not a good idea, as it distorts the figure. The clamps should just stop the mirror from sliding around as it is moved in altitude.

If the figure in the mirror is distorted by the clamps, using higher powers really makes for an impossible image, regardless of quality of eyepiece.

Relax the clamps a little and retry your high power EPs.

I have an 8" f/4 dob. I use a 30 year old Edmund Scientific 28mm RKE eyepiece- no astigmatism. That is because it is designed for use with a fast scope. This is the only 'cheap' eyepiece that is of low power with bugger all astigmatism. At high powers, this little scope is fantastic. I made its mirror cell, and it is held securely, but not tight. I've pushed it to 260X, beautiful. See what I mean.

Oh, this 8" f/4 mirror is also a GSO, .
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